There seems to be some inequity in the implementation of dimension behavior with Revit MEP. When we use RAC and RST we find that we can use temporary or permanent dimensions to reposition most elements.
With RME however Revit does not conduct itself in the same fashion. As an example we need to just look at a pair of ducts that are parallel to each other. A dimension placed between them does not become editable when you select either duct, nor does the Activate Dimensions button appear on the Options Bar. No temporary dimension appears between them.
It is as if it has been determined that behavior that most Revit users have come to expect isn't relevant or appropriate in RME for some reason. The lack of this make it more tedious to place duct work precisely relative to other duct. An exception to this is when two parallel ducts are not connected to each other via another duct.
It is my humble opinion that this deserves another look by developers.